1. Field of the Invention
This invention relates to coatings for turbine blades and particularly to the simultaneous treatment of the internal and external surfaces of turbine blades.
2. Background Art
Today the modern industrial gas turbine operates under conditions that are very aggressive for the nickel and cobalt alloys that are typically used in an engine's hot section. Therefore these alloys are attacked rapidly by the atmosphere in this region of the turbine causing them to degrade and necessitate their premature replacement. The metals that are added to nickel or cobalt alloys to improve the alloys' resistance to corrosive and oxidative environments cannot be added in sufficient concentrations without having a detrimental effect on the alloys' mechanical properties. It is for this reason that protective coatings have been developed thus producing the properties that are required at the surface of the component without having a detrimental effect on the mechanical properties of the base material.
Nowadays the surface engineering solutions used on industrial gas turbines are very diverse and several coating systems may be utilised on an individual turbine blade.
The chemically aggressive environment within land-based power generation gas turbines may lead to corrosion involving alkali and transition metal sulphates at temperatures from 600 to 800° C. (Type II corrosion), corrosion involving molten sulphates from 750 to 950° C. (Type I corrosion), and gaseous oxidation at higher temperatures. Protection of the base material under such conditions is difficult and requires the use of corrosion resistant coatings. Separate coating compositions need to be used for the differing corrosion environments, typically a chromia former (e.g. a chromide diffusion coating) to protect against Type II attack and an alumina former (e.g. an aluminide diffusion coating) for Type I and high temperature attack.
It is standard in the art to employ aluminide coatings to protect turbine blades from high-temperature oxidation and corrosion. It is also currently accepted that enrichment of the surface layer with aluminium provides satisfactory protection against Type I sulphidation. This is the result of the formation of an alumina scale that provides an effective barrier to the penetration of corrosive elements, such as sulphur and oxygen. Chromium cannot be used at the elevated temperatures that are experienced when Type I sulphidation is seen since the oxide scale formed by chromium has a significant vapour pressure at these temperatures. This means that the scale effectively evaporates from the surface and the protection is lost. This is the typical situation observed on the external surface of a gas turbine blade.
At elevated temperatures the turbine blades must be cooled. Cooling may be achieved by forcing compressed air, which may contain sulphur besides oxygen, through cooling channels in the turbine blade. Accordingly, the temperatures experienced on the metal surfaces in this internal region are lower than the temperatures experienced on the external surfaces. Aluminium scales do not form readily at these temperatures where Type II sulphidation occurs and hence aluminium does not provide effective protection against this type of attack. However, chromium oxide scales form readily at this temperature and are also physically stable and hence do provide effective protection against this type of attack.
Therefore the preferred coating system on a turbine blades where Type II sulphidation occurs on the internal surfaces and Type I sulphidation occurs on the external surfaces is aluminium coatings on the external surface and chromium coatings on the internal surfaces.
As well as the turbine blades, the vanes are also made from similar materials to the blades and may also have cooling channels. They are, therefore, subject to similar attacks as the blades.
It is common in the industry that chemical vapour deposition (also termed “diffusion coatings”) is used to apply these protective coatings to industrial gas turbines. In general these coatings are formed when the surface that requires protection is brought into contact with an atmosphere that is rich in the metal to be deposited on the surface. The metal species is usually in the form of a volatile halide. This deposition occurs generally at elevated temperatures (i.e. in excess of 800° C.) and in the presence of a reducing atmosphere, such as hydrogen.
Diffusion coatings of chromium and aluminium are applied in two separate coating runs. However there are several disadvantages to this approach as a viable industrial process. For example, two consecutive processes increases the cost for protecting the turbine blade, it adds significantly to the time that it takes to carry out the process, and the second process to be carried out affects the results of the first coating process.